In the late 1970s, a no water addition lead-acid battery was developed that unlike its predecessor, the Gel battery, used a unique very absorbent separator to hold the battery acid like a sponge. The separator for such an energy storage cell is an absorbent glass mat (AGM) which is a non-woven separator made from spun-glass microfibers. The AGM separator is typically operated in a partially saturated state where acid is absorbed by the separator but the total porosity of the separator is not completely filled with acid. A rechargeable lead-acid energy storage cell containing the AGM separator operates on the principle of oxygen recombination whereby oxygen generated at the positive plate diffuses through the partially saturated AGM separator and is therefore able to be chemically reduced at the surface of the negative electrode to be re-formed back into water. To better facilitate the oxygen recombination reaction and reduce overall water-loss, the energy storage cell also incorporates a pressure relief valve that maintains a low head-pressure (typically from about 1-5 psi) in the cell.
Accordingly oxygen recombination is used to eliminate water addition during the life of the energy storage cell. Because the AGM energy storage cells do not “gas freely” they may also be known as valve regulated lead-acid (VRLA) batteries using AGM technology. As stated above, an AGM separator is a non-woven micro-glass mat separator that is soft, compressible, and very absorbent. In a manner similar to a disposable baby diaper, the AGM separator absorbs and holds the acid. The separator is 92-96% porous and actually absorbs 7-8 times its weight in acid. The AGM energy storage cell is designed so that the thickness of the plates and the AGM separators fit into the cell case very tightly. In fact, most AGM energy storage cells are designed in such a way that the AGM separators are ultimately compressed 20-30% of their uncompressed volume when stuffed into the cell case. Compression of the AGM separator gives the cell the needed plate-to-separator interfacial contact and makes the energy storage cell substantially vibration resistant. The highly porous nature of AGM separators leads to lower internal cell resistance and better high rate performance of the energy storage cell.
In most if not all cases, an AGM energy storage cell is a deep cycle battery that can be used in UPS systems, wheel chairs, portable tools, consumer electronics, alarms, boats, heavy equipment and some toys. Other applications may include emergency lighting, telecommunications equipment, backup power systems and solar powered battery systems.
A problem with AGM energy storage cells is that as the cell ages, the cell loses water and the separator dries out. As the separator dries out, the oxygen recombination rate increases and energy storage cell runs hotter. Under certain conditions, the oxygen recombination in the AGM energy storage cell may become too vigorous causing the energy storage cell to go into thermal runaway. If the charger used for charging the energy storage cell is not temperature compensated, the energy storage cell may eventually melt and, in severe cases, ignite or burn.
Despite the advances made in the art with respect to separators for energy storage cells, there continues to be a need for separators for energy storage cells which exhibit improved physical and electrochemical properties over conventional separators. For example, there is a need for attenuating the oxygen recombination process in an AGM energy storage cell as the cell ages in order to prevent thermal runaway and/or damage to the energy storage cell.
With regard to the above, one embodiment of the disclosure provides a separator for an energy storage cell that is provided by a microporous web that includes an irreversible porosity-controlling agent
Another embodiment provides a method for changing an operating characteristic of an energy storage cell. The method includes applying from about 5 to about 50 weight percent of an irreversible porosity controlling agent to a separator material. An improved separator may be formed from the separator materials and the irreversible porosity-controlling agent. The porosity-controlling agent may be selected from agents that change size as a function of temperature, agents that change size as a function of pH, agents that change size as a function of pressure, and agents that change size as a function of temperature, pH, and/or pressure. The energy storage cell is then operated with the separator.
The separators according to the invention exhibit improved properties as compared to conventional separators. Another advantage of the disclosed embodiments is that the separators may take an active rather than passive role in improving the performance of energy storage cells under variable conditions. Until now, energy storage cell separators have been a passive component of the cells, with the exception of the tri-layer thermal shutdown separator used in the lithium-ion battery industry (see U.S. Pat. No. 5,952,120 and others assigned to Celgard). The “shutdown” separator is a three-layer structure of stretched polypropylene/polyethylene/polypropylene. The internal layer of PE is designed to melt at high temperatures thus increasing the electrical resistance of the storage cell and “shutting down” the energy storage cell. The process of “shutting down” the energy storage cell is irreversible and once this occurs the energy storage cell is non-functional and must be replaced. By comparison, the separators described herein may be used to attenuate the electrical resistance of the separator as the temperature of the energy storage cell rises thereby enabling continued use of the storage cell even as the cell ages and the separator dries out.